A device of this type for determining the replenishment of developer, fixing and water baths based on the detection of the film surface area is, for example, known from DE-PS 31 42 881 by means of which the width of the photosensitive material is detected by a large number of IR transmitters and receivers and the length is detected by determining the transport speed.
DE-PS 31 27 822 discloses an automatically operating replenishing system for processing solutions in a processing unit for photosensitive material in which the surface area of the photosensitive material is calculated by means of a number of sensors sensing the film width and by determining the film transport speed with the aid of a microcomputer.
From DE-PS 25 57 253 it is known that for sensing the length of sheets and web-shaped material a buffer lift is provided at whose direction reversal points switching elements are arranged the time-related actuation of which is indicative of the transport speed of the film material and thus indicative of the length of the material.
All those devices are disadvantageous in that for determining the transport speed of the photosensitive material either the processor produces a signal corresponding to the transport speed by measuring the rotational speed of a shaft or of the drive motor, or the control means itself of the processor delivers such a signal. In both cases the transport speed of the photosensitive material moved through the processor cannot be determined directly and, as a result, an exact determination of the surface area and thus of the amount of the replenisher required is not possible.
For determining the density of the photosensitive material a number of methods are known, e.g. from U.S. Pat. No. 3,623,418 in which a process control strip is initially exposed, developed and then fed to a IR measuring unit for determining the density. The density is compared with a nominal value, deviations from said value causing the valve of a supply tank to open or close for replenishing developer or fixing solution.
It is a disadvantage of the aforementioned method that for maintaining top processing quality of the photographic material process control strips are exposed and developed at periodic intervals so that no continuous control of the processing solutions is possible.
Moreover, the known methods and devices have the disadvantage that photosensitive materials having only very slight differences in density are not clearly detected and thus the processing solutions are not correctly replenished. This is the case, if the optoelectronic sensors in the density measuring unit have to differentiate between "unexposed film" and "no film at all". Due to noise, temperature influences or suchlike, the sensor signals may reach an intensity erroneously indicating the presence of a film of low density. For the same reason, a determination of film width and length may be incorrect. On the one hand, a material might be indicated where there is none and, on the other, a film of low density might not be detected.